Phase-equilibria Evidence for the Bachmann-Bergantz Model of Granitic Melt Segregation: Tests from the Guadalupe Igneous Complex, California

There is uncertainty on how silica-rich melts (70-80% SiO₂) are extracted from their solid residue. The Guadalupe Igneous Complex (GIC), a small bimodal (gabbro-granite) pluton shallowly emplaced at ~149 Ma in the Sierra Nevada arc, may inform on granitic melt production and segregation. In Putirka et al. 2014, we hypothesized GIC granites formed by a mineral-melt extraction process within the meladiorites. However, oxygen isotope analyses of the granite, and the overlying Mariposa formation (M. Paige, unpublished), imply the granite also includes 10-30% partial melt from sedimentary wallrocks, added to the original differentiated meladiorite-derived melts. Regardless of assimilation, if the hybrid liquid equilibrated with the minerals in the meladiorites, then the meladiorites could be cumulates. We use experimentally calibrated mineral-melt equilibria to test whether GIC granitic magmas were in equilibrium with their underlying meladioritic crystalline materials. The granites are very fine-grained, so whole rock compositions should approach liquid compositions, as confirmed by Ratschbacher et al. (2018) based on zircon saturation models. Putirka et al. (2014) show the meladiorites and granites fall on a single major oxide mixing trend; we find that amphibole and plagioclase compositions from the meladiorites are in equilibrium with liquids falling on that same mixing line, but clusters near the granitic endmember. In addition, feldspar compositions predicted to be in equilibrium with a granite liquid, using Putirka (2005), also match plagioclase crystals from the meladiorites. We tentatively conclude that an intermediate "parent" magma contained significant fractions of assimilated country rock, and was not solely the product of fractionation, as originally assumed, but that such an intermediate magma plausibly fractionated into the meladiorites (cumulates) and granites (liquid) by the mineral-melt segregation processes envisioned by Bachmann and Bergantz (2004).